Continuous process and apparatus for electrolytic production of sodium metal from sodium salts

ABSTRACT

Method and apparatus for continuous electrolytic production of sodium metal from sodium salts including a charging cell having feeder launders extending thence to and from a series of electrolysis cells for circulating a feed of molten sodium and other salts from the charging cell through the electrolysis cells for electrolyzing said salts to produce sodium metal collected as such, the electrolysis cells being provided with overflow means for maintaining a constant level of molten electrolyte in each, the electrolyzed sodium and other salts being replenished in said charging cell, the latter including means for maintaining the salt feed in a molten state.

Elite States out 1 9 4mm Love 1 Feb. 8, 11972 [54] CONTINUOUS PROCESS AND 3,203,881 8/1965 Ancrum et al. ...204/69 X APPARATUS FOR ELECTRQLYTHC 3,418,223 12/1968 Love ..204/70 PRODUCTION OF SODHUM METAL Appl. No.: 801,937

Us. Cl. 204/68, 204/237 int. Cl. .CZZd 3/08, BOlk 3/00 Field of Search .204/68-70, 237

References Cited UNITED STATES PATENTS Haynes ..204/70 11/1936 Roder et a1. ..204/68 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Att0meyWard, McElhannon, Brooks and Fitzpatrick and Raymond J. McElhannon S'I'RACT Method and apparatus for continuous electrolytic production of sodium metal from sodium salts including a charging cell having feeder launders extending thence to and from a series of electrolysis cells for circulating a feed of molten sodium and other salts from the charging cell through the electrolysis cells for electrolyzing said salts to produce sodium metal collected as such, the electrolysis cells being provided with overflow means for maintaining a constant level of molten electrolyte in each, the electrolyzed sodium and other salts being replenished in said charging cell, the latter including means for maintaining the salt feed in a molten state. i

9 Claims, 2 Drawing Figures CONTINUOUS PROCESS AND APPARATUS FUR ELECTRQLYTHC PRODUCTKON i SGDllUll/l METAL FROM SQDHUM SALTS This invention pertains to a continuous process and apparatus therefor, for large scale, quantity production of high purity sodium metal by electrolysis of fused salt baths containing sodium salts.

Although the principles of the invention are applicable quite generally to the production of sodium from the various types of fused salt baths from which this element is recovered by electrolysis, the invention is particularly adapted to such recovery from fused baths of metal chloride salts containing sodium chloride and will be described with reference to such.

The production of sodium metal by electrolysis of fused salt baths containing sodium chloride as presently practised commercially, is generally carried out as a batch-type operation in a series of electrolysis cells, into each of which sodium chloride, generally in admixture with other metal chloride salts, is initially charged, usually in a molten state, and subjected to electrolysis by passage of direct current between spaced cathode and anode electrodes immersed in the bath. The resulting electrolysis breaks down the sodium chloride into sodium metal and chlorine gas, which pass to the cathode and anode electrodes respectively, for separate recovery as such.

More specifically, sodium metal may be produced in accordance with current practices by electrolysis of fused mixtures of NaCl and CaCil at operating temperatures of approximately 600 C. The eutectic of NaCl-CaCl contains 66.8 percent CaCl and normally the cell melt for sodium cells will contain from 40 to 42 percent NaCl and 58 to 60 percent CaCl The decomposition potentials for Na and Ca are not greatly different at the operahng conditions, so some calcium is reduced and must later be removed by crystallizing it out of the molten sodium. Sodium chloride is added continuously to maintain a nearly constant level, and calcium chloride is added intermittently to maintain the NaCl- CaCl ratio. The NaCl feed needs to be purified and heated to remove water before charging to maintain high cell efficiency and reduce explosions in the cell. A typical type of fused salt cell produces liquid sodium metal at the cathode, and the metal being lighter than the bath is collected by an inverted trough and flows upward into the collector compartment. The molten sodium from each cell is intermittently removed and the chlorine gas produced at the anode is continuously drawn off from above the melt.

Production as thus currently practised commercially involves operating a large number of individual cells which function independently of one another, each of which has different operating characteristics and which must, therefore, be inde pendently tended by workmen who must periodically withdraw accumulated sodium metal and charge the cells with additional salts. Also adjustments of the cell electrodes must e made from time to time to control cell operating voltages and temperatures. Such batch-type operation of a large bank of cells, therefore, involves high labor and maintenance costs, and rigid scheduling of salt chargings and metal removals. These objectionable feature are eliminated by the continuous mode of operation of the present invention.

In accordance with the present invention, a continuous process and apparatus is provided for circulating a fused salt melt through a bank of electrolysis cells by means of conduits which carry fused melt from a recirculating vessel or vessels to the electrolysis cells, and returns the circulating melt from the cells back to the recirculating vessel or vessels. Circulation can be in series, in parallel or a combination thereof. The circulating cell melt has the following advantages: (a) the ratio of NaCl, CaCl or other melt constituents, can be controlled by adding the required amounts to the recirculating melt and all cells will be operating with the same melt composition; (b) temperature of the melt will be maintained the same in all cells and can be regulated by adding or removing heat from the system; and (c) the recirculating vessel or vessels can be designed so that new feed can be melted and purified with the result that electrolysis cell efficiency is improved and explosions in the cells are eliminated.

Reference will now be had to the accompanying drawing for a more detailed description of the above and other features of the invention, wherein:

FIG. l is a diagrammatic showing in flow sheet arrangement illustrative of the continuous process of the invention and of the essential apparatus assembly therefor; while Fit]. 2 is an enlarged sectional elevation of one of the electrolysis cells of the hi6. ll showing, illustrating the structural details and performance thereof under operating conditions.

Referring to FIG. l, a molten salt feed is formed in a charging cell 15 by directing sodium chloride salt, as at W, together with other metal chloride salts, as at ill, which are added to replenish or adjust the electrolyte, from hoppers l2 and B, respectively, over conveyor l4 into said cell 15, wherein the salts are melted. The charging cell 15 also receives via conduit 20 a circulating flow of partially depleted NaCl electrolyte melt from a series of electrolysis cells E,E,, inc., as hereinafter more fully explained.

Charging cell is equipped with spaced electrodes 16 and i7 energized from an alternating current source 1611, to provide electrical resistance heating means for heating the electrolyte and maintaining it in a molten state. Spaced carbon electrodes 18 and E9 extend into the molten bath and are energized from a low voltage direct current source Heb of up to about 2 volts or so, thereby to purify the melt with respect to oxide impurities having a lower decomposition potential than sodium chloride.

The NaCl containing electrolyte from charging cell 215 is introduced, as by pumping for example from pump 21a, at controlled temperature and composition through conduit 2i into an enclosed, thermally and electrically insulated inclined feeder launder 23 extending along a series of electrolysis cells El E.1m- The molten electrolyte is introduced into these cells vi a fitters? conduits F l F1 extendin g dh wiiwardly from feeder launder 23 to a point below the level of molten electrolyte in each ofthe electrolysis cells E.E,,-. Either continuous or intermittent feeding may be employed, but it is preferred to employ intermittent feeding using small quantities at short intervals to aid in avoiding electrical shorting between cells.

The electrolytic process occurring in each of cells E -E inc., as explained below with reference to FIG. 2, converts the NaCl in the molten salt bath to elemental sodium metal and chlorine gas. The chlorine gas is evolved from the cells via conduits G G inc., connected to a discharge conduit 25. The molten elemental sodium metal is discharged from the electrolysis cells via conduits P P inc. and thence into a delivery conduit 24. The partially depleted cell melt from the electrolysis cells is discharged via conduits 0 -0,, inc. into a refractory lined inclined overflow launder 22, which connects with conduit 2% and thus returns the depleted cell melt to the charging cell 15.

As will be seen from the FIG. 1 drawing, the feeder launder 23 slopes downwardly in the direction away from the charging cell 15 in order to feed the molten salt feed by gravity into the electrolysis cells E,-E inc. successively, via the charging conduit connections F 4 inc. On the other hand, the overflow launder 22 is oppositely inclined in a direction sloping downwardly from the electrolysis cells to the charging cell in order to feed the partially depletcxi molten salt feed by gravity back into the charging cell. As'above explained, the charging cell receives from the supply bins l2 and 13, fresh NaCl salt and other salt additions at rates to offset the depictions thereof occurring in the electrolysis cells.

Referring now to H6. 2, each of the electrolysis cells E -E inc. of HG. ll, comprises in its essentials: an anode 30, preferably of cylindrical configuration, surrounded by an annular cathode 3i spaced therefrom, a collecting chamber 32 and dome 33 integral therewith and disposed above the anode for collecting the chlorine gas, a flanged, annular troughlike cathode collector 34, and perforated metal diaphragms 3S and 36 dependent therefrom on opposite sides respectively of the annular cathode 31 for collection of the molten sodium metal. A riser pipe 37 is disposed integral with the collector 34 above the same for conducting the sodium metal as produced, upwardly and through a connecting pipe 38 into a sodium collecting vessel 39, the sodium metal being lighter than the electrolyte. A pipe 40 into which the pure chlorine gas is discharged from the collecting chamber 32, connects, for each cell, with one of the conduits G,-G,, inc. of FIG. l, as indicated in the FIG. 2 drawing.

The FIG. 2 cell components above described, with the exception of the sodium collecting vessel 39, are disposed within a housing 41 consisting preferably of iron plates lined with a refractory material such as fire brick. The cell housing is closed at the top by means of a cover plate 42 through which extends a conduit 43 to a depth considerably beneath the upper level 44 of the fused elecnrolyte. Above the cover plate, the pipe 33 is connected to one of the conduits F F inc. of FIG. 1 for feeding the fused electrolyte into the cell. The sodium collecting vessel 39 is provided with a valved outlet 45 for discharging the molten metal intermittently via gravity into one of the discharge conduits P P inc. of FIG. H connected thereto, as indicated. The cathode and anode electrodes 30 and 31 have connected thereto as at 46 and 47, conductors which extend through the housing 41 for connecting a direct current voltage source (not shown) therebetween for energizing the cell. The anode 30 is preferably made of graphite or carbon and the cathode 31 of iron or copper.

For maintaining the fused electrolyte in the cell at a substantially constant level 44, there is provided an outlet passage 48 through the cell housing 41 which connects to a thermally insulating conduit 49 having disposed therein a brick overflow weir as at 50 over which any excess of fused electrolyte flows into one of the discharge conduits 4),, inc. connected to the conduit 49 as indicated in FIG. 2.

Operation of this cell is as follows: The direct current energization of the cell, liberates chlorine at the anode and sodium at the cathode. The chlorine rises to the surface of the bath within the dome 33 and into the collecting chamber 32 and is discharged thence via conduit 40 under its own pressure into one of the conduits (l -G, of FIG. 1. The sodium likewise rises in the electrolyte, is caught under the collector 34 and passes upwardly in pipe 37. Since sodium has somewhat lower density than the fused bath, a column of sodium forms and eventually stands high enough in pipe 37 to overflow through conduit 38 into vessel 3%. Continuous production of sodium results in a practically continuous overflow. The bath level is maintained constant by introducing the fused sodium chloride containing feed through conduits 43. There being a large surface exposure of the bath, sodium chloride may be introduced directly into the bath. Any moisture that may be present in the fused salt feed is expelled from the bath and having no way of access to the chlorine and sodium chambers said moisture is ultimately driven tothe outside air.

For practising the invention, any of the conventional fused salt baths containing sodium salts may be employed, such, for example, as those containing a sodium salt and at least one other salt of the alkali and alkaline earth metal salts. The preferred baths, however, are those consisting of sodium chloride and one or more additional chloride salts of the alkali and alkaline earth metals, and in particular, those consisting principally of sodium chloride and lithium chloride, with or without additions of other chloride salts of the above metals, such as CaCl KCl, BaCI etc., and wherein lithium chloride preferably is present in the amount of about 70-8O mole percent.

What is claimed is:

ii. A method for the continuous production of elemental sodium which comprises: forming in a charging cell a molten feed comprising a sodium salt and at least one other salt selected from the group consisting of alkali and alkaline earth metal salts, distributing a portion of said feed melt from said char ng cell into a pluralit of electrolysiscells, each having ove low means for establrs mg and maintaining said melt at a substantially constant level therein, concurrently electrolyzing said melt in said electrolysis cells to form molten sodium and gaseous decomposition products in each and to produce therein an electrolyzed melt which is partially depleted in at least said sodium salt, withdrawing said gaseous decomposition products from said electrolysis cells, separately withdrawing a portion of said molten sodium from said electrolysis cells and separately recirculating the overflow of the electrolyzed melt therefromto said charging cell, and introducing into said charging cell additional amounts of said sodium and other salts to compensate for the depletion thereof in said recirculated electrolyzed melt.

2. The method according to claim 1 wherein said molten feed comprises sodium chloride and at least one other chloride salt selected from the group consisting of alkali and alkaline earth metals.

3. The method according to claim 1 wherein said molten feed comprises sodium chloride and lithium chloride salts.

4. The method according to claim 3 wherein said lithium chloride salt comprises about 70 mole percent of said melt.

5. The method according to claim 1 wherein said salts are maintained molten in said charging cell by passage of alternating current between spaced electrodes immersed therein.

6. The method according to claim 5 wherein oxidic contaminants in the molten salt in said charging cell are eliminated by passage through said solution of direct current between spaced carbon electrodes immersed in said salt.

7. The method according to claim i wherein the molten salt in said electrolysis cells has a density exceeding that of the molten sodium metal for floating the released molten sodium metal thereon. v a

8. The method according to claim 1 wherein said 'molten salt feed is gravity fed from said charging cell to said electrolysis cells and wherein the electrolyzed molten salt in salt cells is gravity fed back to said charging cell.

9. A method for the continuous production of elemental sodium which comprises: forming in a charging cell a molten feed comprising a sodium salt and at least one other salt selected from the group consisting of alkali and alkaline earth metal salts, distributing a portion of said feed melt from said charging cell into a plurality of electrolysis cells, each having overflow means for establishing and maintaining said melt at a substantially constant level therein, concurrently electrolyzing said melt in said electrolysis cells to form molten sodium and gaseous decomposition products in each and to produce therein an electrolyzed melt which is partially depleted in at least said sodium salt, withdrawing said gaseous decomposition products through a collecting chamber and dome disposed above the anode, passing a portion of said molten sodium through perforated metal diaphragms adjacent the cathode, collecting said portion of molten sodium in a troughlike cathode collector disposed above said perforated metal diaphragms, passing said portion of molten sodium upwardly through a riser pipe to a collecting vessel, recirculating the overflow of the electrolyzed melt from said electrolysis cells to said charging cell, and introducing into said charging cell additional amounts of said sodium and other salts to compensate for the depletion thereof in said recirculated electrolyzed melt. 

2. The method according to claim 1 wherein said molten feed comprises sodium chloride and at least one other chloride salt selected from the group consisting of alkali and alkaline earth metals.
 3. The method according to claim 1 wherein said molten feed comprises sodium chloride and lithium chloride salts.
 4. The method according to claim 3 wherein said lithium chloride salt comprises about 70-80 mole percent of said melt.
 5. The method according to claim 1 wherein said salts are maintained molten in said charging cell by passage of alternating current between spaced electrodes immersed therein.
 6. The method according to claim 5 wherein oxidic contaminants in the molten salt in said charging cell are eliminated by passage through said solution of direct current beTween spaced carbon electrodes immersed in said salt.
 7. The method according to claim 1 wherein the molten salt in said electrolysis cells has a density exceeding that of the molten sodium metal for floating the released molten sodium metal thereon.
 8. The method according to claim 1 wherein said molten salt feed is gravity fed from said charging cell to said electrolysis cells and wherein the electrolyzed molten salt in salt cells is gravity fed back to said charging cell.
 9. A method for the continuous production of elemental sodium which comprises: forming in a charging cell a molten feed comprising a sodium salt and at least one other salt selected from the group consisting of alkali and alkaline earth metal salts, distributing a portion of said feed melt from said charging cell into a plurality of electrolysis cells, each having overflow means for establishing and maintaining said melt at a substantially constant level therein, concurrently electrolyzing said melt in said electrolysis cells to form molten sodium and gaseous decomposition products in each and to produce therein an electrolyzed melt which is partially depleted in at least said sodium salt, withdrawing said gaseous decomposition products through a collecting chamber and dome disposed above the anode, passing a portion of said molten sodium through perforated metal diaphragms adjacent the cathode, collecting said portion of molten sodium in a troughlike cathode collector disposed above said perforated metal diaphragms, passing said portion of molten sodium upwardly through a riser pipe to a collecting vessel, recirculating the overflow of the electrolyzed melt from said electrolysis cells to said charging cell, and introducing into said charging cell additional amounts of said sodium and other salts to compensate for the depletion thereof in said recirculated electrolyzed melt. 